Method and system for valve operation control

ABSTRACT

Methods and systems are provided for raising the speed of a hybrid electric vehicle operating in an electric-only mode. During conditions when the vehicle is driven only by an electric motor, vehicle speed may be raised by spinning the engine unfueled using power from a system battery, while adjusting valve operation to reduce engine pumping losses. In this way, vehicle speed may be raised more efficiently and without damaging rotating transmission components.

FIELD

The present application relates to reducing pumping losses whilecontrolling the vehicle speed of a hybrid electric vehicle, inparticular when operating in an electric-only mode.

BACKGROUND AND SUMMARY

Hybrid vehicle systems may be configured with various transmissioncomponents. For example, in power-split transaxle hybrid vehiclesystems, the transmission may include a planetary gear system directlycoupled to the engine, and further coupled to the wheels via one or morerotating components, such as gears, pinions, and bearings. Duringvehicle operation, transmission control takes into consideration thespeed constraints of the various rotating transmission components toavoid damage to the components.

The inventors herein have recognized a potential issue with suchsystems. The mechanically-imposed speed constraints of the transmissioncomponents may limit the maximum attainable vehicle speed, in particularduring engine-off modes of operation. Specifically, when the vehicle isoperated in an electric mode with the engine off, and the vehicle isbeing propelled by energy from a system battery, the maximum allowablevehicle speed is limited to a speed that protects the rotatingtransmission components from speed-incurred damage. Based on theconfiguration of the transmission components, it may be possible to spinthe engine using the battery to reduce the speed-incurred damage to therotating transmission component. However, the battery usage would beincreased by engine parasitic losses, such as pumping losses. Limitedvehicle speed ability may degrade the operator's drive experience whileincreased battery usage may affect the battery's life and performance,as well as degrade the vehicle's fuel economy.

In one example, some of the above issues may be at least partlyaddressed by a method of operating a hybrid vehicle system comprisingduring an engine-off mode with only an electric motor driving thevehicle, raising vehicle speed by increasing the speed of an unfueledengine. The method further comprises adjusting a cylinder valve based onthe engine speed. In this way, airflow through the spinning unfueledengine can be reduced, thereby reducing engine parasitic losses.

For example, a hybrid electric vehicle may be configured with aplanetary gear transmission. During selected conditions when the vehicleis driven only by an electric motor, a controller may allow the vehiclespeed to be increased up to a threshold vehicle speed without spinningthe engine. As such, up to the threshold speed, as the vehicle speedincreases, the rotational speed of a rotating transmission component(such as a gear component) may also increase. Above the thresholdvehicle speed, further rotation of the transmission component may leadto mechanical degradation of the component. Therefore, above thethreshold vehicle speed, the controller may allow the vehicle speed tobe further increased by spinning the engine unfueled. Based on theconfiguration of the transmission component with relation to the engineand the wheels, as the engine speed is increased, the rotational speedof the transmission component may be decreased (or maintained at theupper limit), allowing the maximum vehicle speed attainable in theelectric-only mode to be raised. While the engine is spinning unfueled,the position of a cam phaser may be adjusted to reduce airflow throughthe spinning engine. This may enable the operation of a cylinder valve(e.g., for an intake valve and/or an exhaust valve) to be adjusted. Forexample, a valve timing may be advanced or retarded, as appropriate, toreduce an airflow through the spinning engine. By reducing the airflow,parasitic engine losses, such as pumping losses, may be reduced, therebyallowing the vehicle to continue to be operated using the battery for alonger duration.

In this way, higher vehicle speeds may be attained while continuing tooperate a hybrid vehicle in a fuel-efficient electric mode with powerprovided from a system battery. By adjusting a cylinder valve operationto minimize airflow through the spinning engine, engine pumping lossesmay be reduced. Additionally, oxygen loading of an exhaust catalyst canalso be reduced. By maintaining a rotational speed of a rotatingtransmission component within limits, speed-induced mechanicaldegradation of transmission components may be reduced. By prolonging theelectric mode of vehicle operation without limiting vehicle speed, theoperator's drive experience can be improved while improving the fueleconomy of the vehicle.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example propulsion system for a hybrid electricvehicle.

FIG. 2 illustrates an example internal combustion engine.

FIGS. 3-4 depict high level flow charts for spinning an engine unfueledand adjusting cylinder valve operation during an electric mode of hybridvehicle operation responsive to attain a desired vehicle speed.

FIGS. 5-7 illustrate example scenarios of spinning an engine unfueled toraise a vehicle speed while maintaining the rotational speed of atransmission component at or below a limit.

FIG. 8 depicts a table illustrating the engine status and the source ofvehicle propulsion power during various vehicle operating modes.

FIG. 9 depicts example adjustments to a cylinder intake and/or exhaustvalve operation that may be performed while an engine is spinningunfueled.

DETAILED DESCRIPTION

The following description relates to systems and methods for operating ahybrid vehicle, such as the hybrid electric vehicle (HEV) of FIGS. 1-2.During conditions when the vehicle is only propelled using power from anelectric motor (FIG. 8), vehicle speed may be increased by selectivelyspinning the engine without injecting fuel therein. An engine controllermay be configured to perform a control routine, such as the routine ofFIG. 3, to raise vehicle speed above a threshold speed by spinning theengine unfueled using a system battery, via a generator. By spinning theengine unfueled, the rotational speed of a rotating transmissioncomponent may be maintained within limits, thereby reducing thelikelihood of component degradation. Example operations wherein avehicle speed is raised by spinning an engine unfueled are shown withreference to FIGS. 5-7. The engine controller may be further configuredto perform a control routine, such as the routine of FIG. 4, to adjust acylinder valve operation while the engine is spinning unfueled to reducepumping losses through the engine. Example cylinder valve timingadjustments are shown with reference to FIG. 9.

FIG. 1 depicts a hybrid propulsion system 100 for a vehicle. In thedepicted embodiment, the vehicle is a hybrid electric vehicle (HEV).Propulsion system 100 includes an internal combustion engine 10 having aplurality of cylinders 30. Fuel may be provided to each cylinder ofengine 10 from a fuel system (not shown) including one or more fueltanks, one or more fuel pumps, and injectors 66.

Engine 10 delivers power to transmission 44 via torque input shaft 18.In the depicted example, transmission 44 is a power-split transmission(or transaxle) that includes a planetary gearset 22 and one or morerotating gear elements. Transmission 44 further includes an electricgenerator 24 and an electric motor 26. The electric generator 24 and theelectric motor 26 may also be referred to as electric machines as eachmay operate as either a motor or a generator. Torque is output fromtransmission 44, for propelling vehicle tractions wheels 52, via a powertransfer gearing 34, a torque output shaft 19, and differential-and-axleassembly 36.

Generator 24 is drivably connected to electric motor 26 such that eachof electric generator 24 and electric motor 26 may be operated usingelectric energy from an electrical energy storage device, hereindepicted as battery 54. In some embodiments, an energy conversiondevice, such as an inverter, may be coupled between the battery and themotor to convert the DC output of the battery into an AC output for useby motor. However, in alternate embodiments, the inverter may beconfigured in the electric motor. Due to the mechanical properties ofthe planetary gearset, generator 24 may be driven by a power outputelement (on an output side) of the planetary gearset 22 via mechanicalconnection 32, as further elaborated below.

Electric motor 26 may be operated in a regenerative mode, that is, as agenerator, to absorb energy from vehicle motion and/or the engine andconvert the absorbed kinetic energy to an energy form suitable forstorage in battery 54. Furthermore, electric motor 26 may be operated asa motor or generator, as required, to augment or absorb torque providedby the engine, such as during a transition of engine 10 betweendifferent combustion modes (e.g., during transitions between a sparkignition mode and a compression ignition mode).

Planetary gearset 22 comprises a ring gear 42, a sun gear 43, and aplanetary carrier assembly 46. The ring gear and sun gear may be coupledto each other via the carrier. A first input side of planetary gearset22 is coupled to engine 10 while a second input side of the planetarygearset 22 is coupled to the generator 24. An output side of theplanetary gearset is coupled to vehicle traction wheels 52 via powertransfer gearing 34 including one or more meshing gear elements 60-68.In one example, the meshing gear elements 60-68 may be step ratio gearswherein carrier assembly 46 may distribute torque to the step ratiogears. Gear elements 62, 64, and 66 are mounted on a countershaft 17with gear element 64 engaging an electric motor-driven gear element 70.Electric motor 26 drives gear element 70, which acts as a torque inputfor the countershaft gearing. In this way, the planetary carrier 46 (andconsequently the engine and generator) may be coupled to the vehiclewheels and the motor via one or more gear elements. Hybrid propulsionsystem 100 may be operated in various embodiments including a fullhybrid system, wherein the vehicle is driven by only the engine andgenerator cooperatively, or only the electric motor, or a combination.Alternatively, assist or mild hybrid embodiments may also be employed,wherein the engine is the primary source of torque and the electricmotor selectively adds torque during specific conditions, such as duringa tip-in event. Accordingly, hybrid propulsion system 100 may beoperated in various modes of operation, as depicted in table 800 of FIG.8.

For example, with reference to FIG. 8, the vehicle may be driven in afirst engine-on mode, herein also referred to as an “engine” mode,wherein engine 10 is operated in conjunction with the electric generator(which provides reaction torque to the planetary gear-set and allows anet planetary output torque for propulsion) and used as the primarysource of torque for powering wheels 52 (the generator may also beproviding torque to wheels if in motoring mode). During the “engine”mode, fuel may be supplied to engine 10 from a fuel tank via fuelinjector 66 so that the engine can spin fueled to provide the torque forpropelling the vehicle. Specifically, engine power is delivered to thering gear of the planetary gearset. Coincidentally, the generatorprovides torque to the sun gear 43, producing a reaction torque to theengine. Consequently, torque is output by the planetary carrier to gears62, 64, 66 on countershaft 17, which in turn delivers the power towheels 52. Optionally, the engine can be operated to output more torquethan is needed for propulsion, in which case the additional power isabsorbed by the generator (in generating mode) to charge the battery 54or supply electrical power for other vehicle loads.

In another example, the vehicle may be driven in a second engine-onmode, herein also referred to as an “assist” mode. During the assistmode, engine 10 is operated and used as the primary source of torque forpowering wheels 52 and the electric motor is used as an additionaltorque source to act in cooperation with, and supplement the torqueprovided by, engine 10. During the “assist” mode, as in the engine-onlymode, fuel is supplied to engine 10 so as to spin the engine fueled andprovide torque to the vehicle wheels.

In still another example, the vehicle may be driven in an engine-offmode, herein also referred to as an electric-only mode, whereinbattery-powered electric motor 26 is operated and used as the onlysource of torque for driving wheels 52. As such, during the engine-offmode, no fuel may be injected into engine 10 irrespective of whether theengine is spinning or not. The “engine-off” mode may be employed, forexample, during braking, low speeds, while stopped at traffic lights,etc. Specifically, motor power is delivered to gear element 70, which inturn drives the gear elements on countershaft 17, and thereon driveswheels 52.

Due to the mechanical properties of the planetary gearset and thespecific coupling of different rotating transmission components to thevehicle wheels, the engine, and the battery (via the motor and/orgenerator), during the electric-only mode, as more power is output toincrease the vehicle speed, the rotational speed of a rotatingtransmission component (such as one of the one or more gear elements ofthe transmission) also increases. For example, in the powersplit systemshown, when the ring gear is not spinning (engine not spinning), the sungear 43, shaft 32, and generator 24 must spin in order to allow theplanet carrier assembly to spin (which must spin because the wheels arespinning). The planetary gear-set ratio may require the sun gear to spinproportionally faster than the carrier assembly while engine is stopped.Additionally, the planet gears in the carrier assembly may also spin atspeeds proportionally higher than the carrier itself. Above a thresholdvehicle speed, further rotation of the transmission component may leadto mechanical degradation of the component. As a result, during theelectric-only mode, when the engine is not spinning, the maximumattainable vehicle speed may be limited by the mechanical and rotationalconstraints of the rotating transmission component. However, due to thespecific configuration of the rotating transmission component withrelation to the engine and the wheels, if the engine is spun during theelectric-only mode, without the addition of fuel to the spinning engine,the rotational speed of the transmission component may be decreased (ormaintained at its upper limit) as the speed of engine spinning isincreased. Consequently, the maximum vehicle speed attainable in theelectric-only mode (with the engine spinning unfueled) may be raised(relative to the maximum vehicle speed attainable in the electric-onlymode with the engine not spinning).

Thus, during the engine-off mode, based on the vehicle speed and thedriver demanded torque, the vehicle may be operated in a firstelectric-only mode (Electric_1 mode) wherein the vehicle is propelled bythe battery 54 via the electric motor with the engine not spinning (andnot fueled), or in a second electric-only mode (Electric_2 mode) whereinthe vehicle is propelled by the battery 54 via the electric motor withthe engine spinning unfueled. During the second electric-only mode, thegenerator applies torque to the planetary gear-set 22 through sun gear43. The planet carrier provides reaction torque to this generatortorque, and consequently directs torque to the engine to spin theengine. In the proposed embodiment, the reaction torque provided by thecarrier is supplied by the motor 26 (or alternatively vehicle momentumduring deceleration events), and consequently reduces torque from to themotor to the wheels. For this reason, motor torque can be increasedduring such events so that no disruption in wheel torque is observed bythe driver. The cooperative supply of torque to the engine by both themotor 26 and generator 24 is what spins the engine and preventstransaxle components from over-speeding.

Returning to FIG. 1, propulsion system 100 may further include a controlsystem including controller 12 configured to receive information from aplurality of sensors 16 (various examples of which are described herein)and sending control signals to a plurality of actuators 81 (variousexamples of which are described herein). As one example, sensors 16 mayinclude various pressure and temperature sensors, a fuel level sensor,various exhaust gas sensors, etc. The various actuators may include, forexample, the gear set, cylinder fuel injectors (not shown), an airintake throttle coupled to the engine intake manifold (not shown), etc.Controller 12 may receive input data from the various sensors, processthe input data, and trigger the actuators in response to the processedinput data based on instruction or code programmed therein correspondingto one or more routines. Example control routines are described hereinwith regard to FIGS. 3-4.

FIG. 2 depicts an example embodiment of a combustion chamber or cylinderof engine 10 (of FIG. 1). Engine 10 may receive control parameters froma control system including controller 12 and input from a vehicleoperator 130 via an input device 132. In this example, input device 132includes an accelerator pedal and a pedal position sensor 134 forgenerating a proportional pedal position signal PP. Cylinder (hereinalso “combustion chamber”) 30 of engine 10 may include combustionchamber walls 136 with piston 138 positioned therein. Piston 138 may becoupled to crankshaft 140 so that reciprocating motion of the piston istranslated into rotational motion of the crankshaft. Crankshaft 140 maybe coupled to at least one drive wheel of the passenger vehicle via atransmission system. Further, a starter motor may be coupled tocrankshaft 140 via a flywheel to enable a starting operation of engine10. Specifically, the generator 24 and driveline including motor 26 arecoupled to the crankshaft and provide torque for engine cranking.

Cylinder 30 can receive intake air via a series of intake air passages142, 144, and 146. Intake air passage 146 can communicate with othercylinders of engine 10 in addition to cylinder 30. In some embodiments,one or more of the intake passages may include a boosting device such asa turbocharger or a supercharger. For example, FIG. 2 shows engine 10configured with a turbocharger including a compressor 174 arrangedbetween intake passages 142 and 144, and an exhaust turbine 176 arrangedalong exhaust passage 148. Compressor 174 may be at least partiallypowered by exhaust turbine 176 via a shaft 180 where the boosting deviceis configured as a turbocharger. However, in other examples, such aswhere engine 10 is provided with a supercharger, exhaust turbine 176 maybe optionally omitted, where compressor 174 may be powered by mechanicalinput from a motor or the engine. A throttle 20 including a throttleplate 164 may be provided along an intake passage of the engine forvarying the flow rate and/or pressure of intake air provided to theengine cylinders. For example, throttle 20 may be disposed downstream ofcompressor 174 as shown in FIG. 2, or alternatively may be providedupstream of compressor 174.

Exhaust passage 148 can receive exhaust gases from other cylinders ofengine 10 in addition to cylinder 30. Exhaust gas sensor 128 is showncoupled to exhaust passage 148 upstream of emission control device 178.Sensor 128 may be selected from among various suitable sensors forproviding an indication of exhaust gas air/fuel ratio such as a linearoxygen sensor or UEGO (universal or wide-range exhaust gas oxygen), atwo-state oxygen sensor or EGO (as depicted), a HEGO (heated EGO), aNOx, HC, or CO sensor, for example. Emission control device 178 may be athree way catalyst (TWC), NOx trap, various other emission controldevices, or combinations thereof.

Exhaust temperature may be estimated by one or more temperature sensors(not shown) located in exhaust passage 148. Alternatively, exhausttemperature may be inferred based on engine operating conditions such asspeed, load, air-fuel ratio (AFR), spark retard, etc. Further, exhausttemperature may be computed by one or more exhaust gas sensors 128. Itmay be appreciated that the exhaust gas temperature may alternatively beestimated by any combination of temperature estimation methods listedherein.

Each cylinder of engine 10 may include one or more intake valves and oneor more exhaust valves. For example, cylinder 30 is shown including atleast one intake poppet valve 150 and at least one exhaust poppet valve156 located at an upper region of cylinder 30. In some embodiments, eachcylinder of engine 10, including cylinder 30, may include at least twointake poppet valves and at least two exhaust poppet valves located atan upper region of the cylinder.

Intake valve 150 may be controlled by controller 12 by cam actuation viacam actuation system 151. Similarly, exhaust valve 156 may be controlledby controller 12 via cam actuation system 153. Cam actuation systems 151and 153 may each include one or more cams and may utilize one or more ofcam profile switching (CPS), variable cam timing (VCT), variable valvetiming (VVT) and/or variable valve lift (VVL) systems that may beoperated by controller 12 to vary valve operation. The position ofintake valve 150 and exhaust valve 156 may be determined by valveposition sensors 155 and 157, respectively. In alternative embodiments,the intake and/or exhaust valve may be controlled by electric valveactuation. For example, cylinder 30 may alternatively include an intakevalve controlled via electric valve actuation and an exhaust valvecontrolled via cam actuation including CPS and/or VCT systems. In stillother embodiments, the intake and exhaust valves may be controlled by acommon valve actuator or actuation system, or a variable valve timingactuator or actuation system.

As such, air flow requirements may differ when the engine is spinningfueled (to provide torque to drive the vehicle) as compared to when theengine is spinning unfueled (to assist in keeping the rotational speedof a rotating transmission component within limits). In particular, ahigher air flow may be needed when the engine is spun fueled, forexample, during an engine-on mode when the engine is started and avehicle speed is raised, while a lower air flow may be provided when theengine is spun unfueled, for example, during an electric-only mode ofoperation when the motor drives the vehicle and the engine is spun toraise the vehicle speed above a threshold vehicle speed. To reduce airflow (and corresponding pumping losses) through the engine when theengine is spinning unfueled during the electric-only mode, a cylindervalve operation (e.g., timing, lift, duration of opening, amount ofoverlap, etc.) may be adjusted. In one example, where the cylindervalves are cam-actuated valves, the position of a cam phaser may beadjusted (e.g., advanced or retarded) to thereby adjust the valveoperation. As elaborated herein with reference to FIGS. 4 and 9, theposition of a cam phaser may be adjusted from a first position (enablinghigher air flow) to a second position (enabling lower airflow) incoordination with the increase in engine speed when the engine isspinning unfueled.

Cylinder 30 can have a compression ratio, which is the ratio of volumeswhen piston 138 is at bottom center to top center. Conventionally, thecompression ratio is in the range of 9:1 to 10:1. However, in someexamples where different fuels are used, the compression ratio may beincreased. This may happen, for example, when higher octane fuels orfuels with higher latent enthalpy of vaporization are used. Thecompression ratio may also be increased if direct injection is used dueto its effect on engine knock.

In some embodiments, each cylinder of engine 10 may include a spark plug192 for initiating combustion. Ignition system 190 can provide anignition spark to combustion chamber 30 via spark plug 192 in responseto spark advance signal SA from controller 12, under select operatingmodes. However, in some embodiments, spark plug 192 may be omitted, suchas where engine 10 may initiate combustion by auto-ignition or byinjection of fuel as may be the case with some diesel engines.

In some embodiments, each cylinder of engine 10 may be configured withone or more injectors for providing a knock or pre-ignition suppressingfluid thereto. In some embodiments, the fluid may be a fuel, wherein theinjector is also referred to as a fuel injector. As a non-limitingexample, cylinder 30 is shown including one fuel injector 166. Fuelinjector 166 is shown coupled directly to cylinder 30 for injecting fueldirectly therein in proportion to the pulse width of signal FPW receivedfrom controller 12 via electronic driver 168. In this manner, fuelinjector 166 provides what is known as direct injection (hereafter alsoreferred to as “DI”) of fuel into combustion cylinder 30. While FIG. 2shows injector 166 as a side injector, it may also be located overheadof the piston, such as near the position of spark plug 192. Such aposition may improve mixing and combustion when operating the enginewith an alcohol-based fuel due to the lower volatility of somealcohol-based fuels. Alternatively, the injector may be located overheadand near the intake valve to improve mixing.

Fuel may be delivered to fuel injector 166 from a high pressure fuelsystem 8 including fuel tanks, fuel pumps, and a fuel rail.Alternatively, fuel may be delivered by a single stage fuel pump atlower pressure, in which case the timing of the direct fuel injectionmay be more limited during the compression stroke than if a highpressure fuel system is used. Further, while not shown, the fuel tanksmay have a pressure transducer providing a signal to controller 12. Itwill be appreciated that, in an alternate embodiment, injector 166 maybe a port injector providing fuel into the intake port upstream ofcylinder 30.

As described above, FIG. 2 shows only one cylinder of a multi-cylinderengine. As such each cylinder may similarly include its own set ofintake/exhaust valves, fuel injector(s), spark plug, etc.

Fuel tanks in fuel system 8 may hold fuel with different qualities, suchas different compositions. These differences may include differentalcohol content, different octane, different heat of vaporizations,different fuel blends, and/or combinations thereof etc. In one example,fuels with different alcohol contents could include one fuel beinggasoline and the other being ethanol or methanol. In another example,the engine may use gasoline as a first substance and an alcoholcontaining fuel blend such as E85 (which is approximately 85% ethanoland 15% gasoline) or M85 (which is approximately 85% methanol and 15%gasoline) as a second substance. Other alcohol containing fuels could bea mixture of alcohol and water, a mixture of alcohol, water and gasolineetc.

Controller 12 is shown in FIG. 2 as a microcomputer, includingmicroprocessor unit 106, input/output ports 108, an electronic storagemedium for executable programs and calibration values shown as read onlymemory chip 110 in this particular example, random access memory 112,keep alive memory 114, and a data bus. Controller 12 may receive varioussignals from sensors coupled to engine 10, in addition to those signalspreviously discussed, including measurement of inducted mass air flow(MAF) from mass air flow sensor 122; engine coolant temperature (ECT)from temperature sensor 116 coupled to cooling sleeve 118; a profileignition pickup signal (PIP) from Hall effect sensor 120 (or other type)coupled to crankshaft 140; throttle position (TP) from a throttleposition sensor; absolute manifold pressure signal (MAP) from sensor124, cylinder AFR from EGO sensor 128, and abnormal combustion from aknock sensor. Engine speed signal, RPM, may be generated by controller12 from signal PIP. Manifold pressure signal MAP from a manifoldpressure sensor may be used to provide an indication of vacuum, orpressure, in the intake manifold.

Storage medium read-only memory 110 can be programmed with computerreadable data representing instructions executable by processor 106 forperforming the methods described below as well as other variants thatare anticipated but not specifically listed.

Now turning to FIG. 3, an example routine 300 is described for raising avehicle speed during an electric-only mode of operation whilemaintaining the rotational speed of a rotating transmission componentwithin limits by spinning an engine unfueled.

At 302, the routine includes estimating and/or measuring one or morevehicle operating parameters such as brake pedal position, acceleratorpedal position, battery state of charge, engine temperature, ambienttemperature and humidity, barometric pressure, etc. At 304, a vehiclemode of operation may be determined based on the estimated operatingconditions. For example, based at least on the estimated driver torquedemand and the battery state of charge, it may be determined whether thevehicle is to be operated in an engine mode (with the engine driving thevehicle wheels), an assist mode (with the battery assisting the enginein driving the vehicle), or an electric-only mode (with only the batterydriving the vehicle). In one example, if the demanded torque can beprovided by only the battery, the vehicle may be operated in theelectric-only mode. In another example, if the demanded torque cannot beprovided by the battery, the vehicle may be operated in the engine mode,or in the assist mode. The vehicle may accordingly be operated in thedetermined mode of operation.

At 306, it may be confirmed that the vehicle is in the electric-onlymode. As such, the vehicle system may include a battery coupled tovehicle wheels via a motor, wherein the electric-only mode of operationincludes propelling the vehicle using the battery via the motor. Thatis, without operating the engine. Additionally, electric-only operationmay include decelerative events, whereby the vehicle is decelerating byvirtue of its own parasitic loads (tire losses, aerodynamic losses, etc)or by using the electric motor to perform regenerative braking. If theelectric-only mode is not confirmed, the routine may end.

Upon confirmation, at 308, while the electric motor is being used forpropulsion, it may be determined whether the vehicle speed is at orabove a threshold vehicle speed. The threshold vehicle speed maycorrespond to a vehicle speed above which a rotating transmissioncomponent may degrade. The threshold vehicle speed may optionally beadjusted based on the battery state of charge. In one example, thethreshold vehicle speed is 62 mph when the battery is fully charged.During the electric-only mode of vehicle operation, as the motor isoperated to drive the vehicle wheels and increase the vehicle speed, therotational speed of the rotating transmission component (ratiometricallytied to wheel speed) may also increase until the component reachesmechanical constraints when the vehicle is at or above the thresholdvehicle speed. Herein, to reduce the risk of mechanical degradation ofthe rotating transmission component, an engine control system may spinthe engine unfueled for at least a duration of the electric-only mode ofvehicle operation when operating the vehicle in the electric-only modeat vehicle speeds above the threshold vehicle speed. It will beappreciated that in some embodiments, the threshold vehicle speed may befurther adjusted based on a temperature of the rotating transmissioncomponent. For example, as the component temperature increases, thethreshold vehicle speed may be lowered to reduce thermal damage to thetransmission component.

At 310, and as elaborated in FIG. 4, in response to the vehicle speedbeing at or above the threshold speed, the vehicle may be operated inthe electric-only mode with the engine spinning unfueled to maintain arotational speed of the rotating transmission component at or below arotational speed limit (such as in electric_2 mode of FIG. 8). Herein,spinning the engine unfueled may include increasing the engine speed(without injecting fuel to the engine) in proportion to the raising ofthe vehicle speed. In comparison, at 312, in response to the vehiclespeed being below the threshold speed, the vehicle may be operated inthe electric-only mode without spinning the engine to maintain therotational speed of the rotating transmission component at or below therotational speed limit (such as in electric_1 mode of FIG. 8).

In this way, during an electric-only mode of vehicle operation, whenbelow a threshold vehicle speed, a controller may raise vehicle speedwithout spinning the engine while increasing a rotational speed of atransmission component. Then, when above the threshold vehicle speed,the controller may raise vehicle speed without increasing the rotationalspeed of the transmission component by spinning the engine unfueled.

Now turning to FIG. 4, an example routine 400 is described for adjustingan engine speed at which to spin the engine unfueled during an electricmode of vehicle operation. The method further describes adjustments to acylinder valve operation during the unfueled engine spinning to reduceparasitic engine losses.

At 402, the vehicle speed may be estimated. At 404, the rotational speedof the rotating transmission component may be determined. In oneexample, the rotational speed may be estimated by a speed sensor. Inanother example, the rotational speed may be inferred based on therotational speed of one or more of the vehicle wheels, the motor, thegenerator, the one or more intermediate gear elements, the connectingshafts (input shaft, output shaft, or countershaft), and the gear ratiosbetween the components. To reduce the rotational speed of the rotatingtransmission component, or to maintain the rotational speed of thecomponent at or below a threshold rotational speed, a controller mayspin the engine unfueled at an engine speed that is based on at leastthe vehicle speed.

Accordingly, at 406, based on the determined vehicle speed and furtherbased on the speeds and limitations of the rotating components of theplanetary gear transmission, an engine speed setting for spinning theengine unfueled may be determined. In one example, spinning the engineat an engine speed based on the vehicle speed may include spinning theengine at an engine speed based on a degree to which the vehicle speedis above the threshold vehicle speed. Thus, as the vehicle speed risesabove the threshold speed, the engine speed may be increased. The enginespeed may be increased in proportion to, or as an alternate function of,the difference between the estimated vehicle speed and the thresholdvehicle speed.

As such, spinning the engine unfueled may include operating thegenerator to spin the engine at the selected engine speed, the generatoroperated using power from the battery. Accordingly, at 408, generatorsettings required to spin the engine unfueled at the selected enginesetting may be determined. In some embodiments, each of the generatorand the motor may be operated to spin the engine at the selected enginespeed while rotating the transmission component at or below thethreshold rotational speed. In those embodiments, generator and motorsettings required to spin the engine unfueled at the selected enginesetting may be determined. The engine speed may be set to be acalibrateable speed that is stored in the controller's memory in alook-up table accessed as a function of the vehicle speed.Alternatively, the engine speed setting may be a minimum engine speedsetting required to meet the transmission component constraints, theengine speed setting continually updated based on the vehicle operatingconditions. In one example, the engine speed setting selected forspinning the engine unfueled during the electric-only mode may be lowerthan an engine speed setting required when spinning the engine fueledduring an engine-only mode to meet the same amount of driver demandedtorque. Herein, by reducing the engine speed at which the engine isspun, power losses required to spin the engine may also be reduced.Additionally, resonance of a damper is avoided, in particular at 400-500rpm.

At 410, the engine may be spun unfueled. Specifically, the generator maybe operated to spin the engine at the selected engine setting while nofuel is injected into the engine cylinders. In this way, a vehicle speedmay be raised above the threshold vehicle speed by spinning the engineunfueled when the vehicle is operated in the electric-only mode.

At 412, while raising the vehicle speed by increasing the speed of theunfueled engine, a cylinder valve operation may be adjusted based on theengine speed, if possible. As such, the cylinder valve adjustment mayenable airflow through the engine to be reduced (e.g., minimized) whenthe engine is spinning unfueled so as to reduce parasitic engine losses,such as pumping losses, and to reduce air flow through the exhaustcatalyst. As such, pumping losses may occur whenever cylinder intake orexhaust valves are open because air flowing into or out of the cylinderhas associated flow losses due to flow restrictions, turbulence, etc.For most engines, valve opening/closing cannot be completely eliminated.However, for engines operating with variable valve timing (VVT), pumpinglosses can be reduced (e.g., minimized) by opening and closing thevalves in a way that reduces flow restrictions. For example, byincreasing valve overlap between exhaust valve opening and intake valveopening, the restriction on air flow may be reduced, thereby reducingpumping losses.

As used herein, the cylinder valve may be an intake valve and/or anexhaust valve and adjusting the cylinder valve operation may includeadjusting one or more of a valve lift, a valve timing, a duration ofvalve opening, a valve-open dwell time, and an amount of valve overlap.As used herein, adjusting the valve timing may include adjusting atiming of valve opening and/or a timing of valve closing. In oneexample, where the cylinder valve is a cam-actuated valve, adjusting thecylinder valve operation may include adjusting a position of a camphaser coupled to (a cam of) the cylinder valve. In another example,where the cylinder valve is an electrically-actuated valve, the cylindervalve operation may be substantially immediately performed. Exampleadjustments of cylinder valve operation are elaborated below withreference to FIG. 9.

Unfueled engine spinning may then be continued for a duration based onthe driver demand and/or a battery state of charge. For example, aselaborated below and in the examples of FIGS. 5-7, the engine maycontinue to be spun with no fuel injected into the engine, while thevehicle is propelled by the electric motor, until the driver torquedemand changes substantially or until the battery state of charge dropsbelow a threshold state of charge (below which the battery may not beable to support the torque demand).

Thus, at 414, it may be determined if there is a change in driver torquedemand that necessitates an engine (re)start. In one example, inresponse to a driver torque demand being higher than a threshold amount,an engine start may be required. Specifically, the higher torque demandmay correlate with a desired vehicle speed that is substantially higherthan the threshold vehicle speed. For example, even with valve timingadjustments, the combination of spinning the engine unfueled andoperating the motor may not be fuel economical and/or mechanicallyfeasible. During such conditions, the desired torque may be betterprovided by operating the engine fueled.

If no engine start is requested, the routine may end. If an engine startis requested, then at 416, cylinder valve operation adjustments may bemade to increase airflow and enable the engine start. For example, a camphaser coupled to cylinder intake and/or exhaust valves may berepositioned to increase airflow through the engine and enable an enginerestart. At 418, following the cylinder valve adjustments, fuel andspark may be returned to the already spinning engine to enable enginestarting for further increasing the vehicle speed via engine torque. Inone example, the engine may be pre-synchronized so that is it ready forignition and fuel injection. Upon engine start, usage of the electricmotor for propulsion may be suspended (if the entire torque demand is tobe provided without motor). Note however, in this powersplitconfiguration shown by FIG. 1, the generator must still operate toprovide reaction torque to the engine, so that a net torque is outputfrom the planetary gear-set (generator can operate in generating mode ormotoring mode to provide this reaction torque). Alternatively, electricmotor operation may be continued if motor torque is required to assistthe engine torque in meeting the driver torque demand. That is, inresponse to the increased torque and/or vehicle speed demand, thevehicle may be operated in an engine-on mode immediately following thepreceding engine-off (or electric-only) mode of vehicle operation. Inparticular, after operating above the threshold vehicle speed with theengine spinning unfueled for a duration, a controller may fuel and startthe engine to maintain the vehicle speed above the vehicle speedthreshold, or to further increase the vehicle speed above a second,higher speed threshold.

In this way, a method is provided for operating a hybrid vehicle systemincluding an engine, a planetary gear transmission, and abattery-operated motor, by which method the upper limit of a vehiclespeed that can be attained during an electric-only mode of operation canbe raised. The method comprises, during a first condition, when thehybrid vehicle system is operated in an engine-on mode with or withoutoperating the motor, raising vehicle speed above a threshold vehiclespeed by operating the engine fueled. In comparison, during a secondcondition, when the hybrid vehicle system is operated in an electricmode, raising vehicle speed above the threshold vehicle speed byspinning the engine unfueled while operating the motor. Herein, duringthe second condition, when the vehicle speed is below the thresholdspeed, a rotational speed of a rotating transmission component of theplanetary gear transmission may increase as the vehicle speed increases,and when the vehicle speed is above the threshold speed, the rotationalspeed of the rotating transmission component of the planetary geartransmission may decrease as the engine speed increases. Further, duringthe second condition, in response to an increase in torque demand, thealready spinning engine is fueled and started to further raise thevehicle speed while disabling operation of the motor.

In one example, once the vehicle speed exceeds 62 mph, instead ofrunning the engine fueled at 1000 rpm (e.g., at a stable combustionspeed), the engine speed may be increased to 600 rpm and the engine maybe spun unfueled. The fuel may remain shut off until the vehicle speedexceeds 72 mph, after which fuel and spark may be returned to the engineto further raise the vehicle speed. Herein, by spinning the engineunfueled at 600 rpm, vehicle operation may be continued withoutinjecting fuel up to a speed of 72 mph, which covers the vehicle speedrange applicable in a majority of highways in the US. As such, since thecrankshaft power requirement at the lower engine speed setting (e.g.,1.1 kW of crankshaft power at 600 rpm) is lower than the crankshaftpower requirement at the higher engine speed setting (e.g., 2.2 kW ofcrankshaft power at 1000 rpm), this approach may improve the fueleconomy of a hybrid vehicle during the first 30-40 miles of a vehiclecommute starting from a full charge between the speeds of 62 mph and 72mph.

The concepts introduced in the routines of FIGS. 3-4 are now furtherclarified by the illustrative examples of FIGS. 5-7.

FIG. 5 shows a first example 500 wherein changes in vehicle speed (Vs)are depicted at graph 502, changes in an engine speed (Ne) are depictedat graph 506, and changes in the rotational speed of a rotatingtransmission component (Ncomp) are depicted at graph 510, over aduration of vehicle operation (along the x-axis). As such, over theentire duration of operation depicted in FIG. 5, the vehicle is in anelectric-only mode of operation and no fuel is injected into the engine.

Between t1 and t2, in response to a driver torque demand, a vehiclespeed may be gradually increased (graph 502) by providing propulsionpower to the vehicle wheels from an electric motor. Due to the specificcoupling of the transmission components, as the vehicle speed increases,while the vehicle speed is below threshold vehicle speed 503, therotational speed (graph 510) of a rotating transmission component thatis connected with the motor and the wheels via one or more gearelements, and is further coupled to the engine via a planetary gearset,may increase. In the depicted example, the rotational speed of thetransmission component is shown increasing in direct proportion to theincrease in vehicle speed while engine is not spinning. However in otherexamples, the rotational speed of the transmission component mayincrease as an alternate function of the increase in vehicle speed.

At t2, the vehicle speed may reach the threshold vehicle speed 503 whichcorresponds to a vehicle speed at which the rotation transmissioncomponent reaches a threshold rotational speed 511. As such, thetransmission component may be degraded if the rotational speed isallowed to increase above the threshold rotational speed 511. Therefore,in the absence of any adjustments to an engine speed, the vehicle speedmay not be allowed to exceed threshold vehicle speed 507. That is, ifoperation is continued in the electric-only mode without spinning theengine (see dashed graph 508 after t2), the rotational speed of thetransmission component may be maintained at or below thresholdrotational speed 511 (see dashed graph 512 after t2) by limiting thevehicle speed to the threshold vehicle speed 503 (see dashed graph 504after t2). The inventors herein have identified that due to the specificcoupling of the engine and motor to the wheels via the planetary geartransmission and the generator, vehicle speed may be raised during anengine-off mode with only the electric motor driving the vehicle andwithout increasing the rotational speed of the transmission component byincreasing the speed of an unfueled engine.

Thus, at t2, a controller may increase the speed of the unfueled engineby spinning the engine using a generator coupled to the engine, thegenerator powered using energy from a system battery. As a result,between t2 and t3, the rotational speed of the rotating transmissioncomponent decreases as the rotational speed of the unfueled engineincreases. After t3, the vehicle speed may be maintained at the highervehicle speed, while the transmission component is kept below thethreshold rotational speed, by maintaining the engine spinning at aselected speed setting, the selected speed setting based on the currentvehicle speed.

As such, it will be appreciated that for the entire duration shown inFIG. 5, the engine may be spinning with no fuel injected into thecylinders. However, in response to a driver torque demand becominghigher than a threshold amount while the vehicle is in the electric-only(or engine-off) mode, the torque demand may be met by shifting thevehicle to an engine-on mode of operation. Specifically, the controllermay return fuel and spark to the already spinning engine to furtherincrease the vehicle speed and further increase the rotational speed ofthe engine. In one example, after operating the vehicle at or above thethreshold vehicle speed with the engine spinning unfueled for aduration, the controller may fuel the engine to maintain the vehiclespeed above the threshold vehicle speed or to further increase thevehicle speed.

FIG. 6 shows a second example 600 wherein changes in vehicle speed (Vs)are depicted at graph 602, changes in an engine speed (Ne) are depictedat graph 606, and changes in the rotational speed of the rotatingtransmission component (Ncomp) are depicted at graph 610, over aduration of vehicle operation (along the x-axis). As such, over theentire duration of operation depicted in FIG. 6, the vehicle is in anelectric-only mode of operation and no fuel is injected into the engine.

In the example illustrated in FIG. 6, rather than limiting the highestvehicle speed that can be attained when operating the vehicle in anengine-off mode (with only the electric motor driving the vehicle), thevehicle may be allowed to operate at vehicle speeds higher than thethreshold vehicle speed for a duration, after which, the engine may bespun unfueled to maintain or further raise the vehicle speed. That is, ashort-term over-speeding may be allowed, but the engine may then be spununfueled if the elevated vehicle speed is sustained. Specifically,between t11 and t12, in response to a driver torque demand, the vehiclespeed may be gradually increased (graph 602) by providing propulsionpower to the vehicle wheels from the electric motor, with a concomitantincrease in the rotational speed (graph 610) of the rotatingtransmission component due to the specific coupling of the vehiclecomponents. At the same time, the engine may not be spinning (graph606).

At t12, the vehicle speed reaches the threshold vehicle speed 503 whilethe rotating transmission component reaches the threshold rotationalspeed 511. After t12, the vehicle speed may be raised above thresholdvehicle speed 503 without increasing rotational speed of the engine fora duration d1 (between t12 and t13). Over the same duration d1, therotational speed of the transmission component may be maintained at thethreshold rotational speed 511. The duration may be based on the vehiclespeed, in particular, based on a difference between the (current)vehicle speed and the threshold vehicle speed (herein depicted as ΔVs).Thus, as the vehicle speed increases above the threshold vehicle speed(that is, as ΔVs increases), the duration may be decreased. Then at t13,after the duration d1 has elapsed, vehicle speed may be maintained orfurther raised by increasing the speed of the unfueled engine (graph606) in proportion to the raising of the vehicle speed. Specifically,between t13 and t14, the engine may be spun using power from a battery(via a generator). In this way, the rotational speed of the rotatingtransmission component may be decreased or maintained below thethreshold rotational speed by increasing the speed of the unfueledengine. Thereafter (after t14), vehicle speed may be maintained (at theelevated vehicle speed) by maintaining the engine speed while alsomaintaining the rotational speed of the transmission component.

FIG. 7 shows a third example 700 wherein changes in vehicle speed (Vs)are depicted at graph 702, changes in an engine speed (Ne) are depictedat graph 706, and changes in the rotational speed of the rotatingtransmission component (Ncomp) are depicted at graph 710, over aduration of vehicle operation (along the x-axis). As such, over theentire duration of operation depicted in FIG. 7, the vehicle is in anelectric-only mode of operation and no fuel is injected into the engine.

Between t21 and t22, in response to a driver torque demand, the vehiclespeed may be gradually increased (graph 702) by providing propulsionpower to the vehicle wheels from the electric motor, with a concomitantincrease in the rotational speed (graph 710) of the rotatingtransmission component due to the specific coupling of the vehiclecomponents. At the same time, the engine may not be spinning (graph706). At t22, the vehicle speed reaches the threshold vehicle speed 503while the rotating transmission component reaches the thresholdrotational speed 511. Hereafter, vehicle speed may be raised abovethreshold vehicle speed 503 by spinning the engine unfueled using abattery.

As previously elaborated in FIG. 4, during the electric-only mode ofoperation, while spinning the engine unfueled using a battery, acontroller may position a cam phaser coupled to intake and/or exhaustvalves of the engine to reduce airflow through the spinning engine.Thus, after t22, the engine may be spun unfueled with the spinning ofthe engine coordinated with a cam phaser repositioning and cylindervalve operation adjustment. In one example, adjusting the position ofthe cam phaser may include advancing or retarding the cam phaser from afirst position having a higher air flow through the spinning engine to asecond position having lower air flow through the spinning unfueledengine. The first position may be a default position that is used tomaximize air flow through the engine during engine restarts. Incomparison, to reduce engine pumping losses through the unfueledspinning engine, the cam phaser may need to be repositioned to thesecond position with lower airflow through the engine, but reduced airflow restrictions. The second position may be based on the (current)rotational speed of the engine. In another example, the first and secondpositions may be adjusted so that pumping losses are reduced (byreducing flow restrictions) while also reducing air flow through anexhaust catalyst so as to maintain catalyst temperature. In the depictedexample, the cam phaser may be coupled to a mechanical oil pump whereina pump output (flow, speed, pressure, etc.) is based on the enginespeed. Thus, as the engine speed increases, the pump output may beconfigured to correspondingly increase. Consequently, the adjusting of aposition of the cam phaser may be delayed until an output of themechanical oil pump is above a threshold pressure. In particular,between t22 and t23, the cam phaser may be maintained at the first(default) position (having a higher airflow) while the engine is spun(graph 706) until the speed of the unfueled engine is above a thresholdengine speed 705 at which the output of the mechanical oil pump is abovethe threshold pressure. Then at t23, after the engine speed is above thethreshold engine speed, the cam phaser may be shifted to the second(desired) position (having a lower airflow).

As such, between t22 and t23, while an engine speed is increased, therotational speed of the rotating transmission component maycorrespondingly decrease. Then, between t23 and t24, while and after thecam phaser is repositioned, the vehicle speed may be increased whilemaintaining the engine speed at threshold engine speed 705 and while therotational speed of the rotating transmission component correspondinglyincreases. At t24, the rotational speed of the transmission componentmay again reach threshold rotational speed 511. Accordingly, between t24and t25, the engine speed may be increased, without the addition of fuelto the engine, to allow the vehicle speed to further increase whileenabling the rotational speed of the transmission component to decreasebelow threshold rotational speed 511. Specifically, increasing the speedof the unfueled engine between t23 and t25 includes spinning the engineto increase engine speed at a first, higher rate (as determined by theslope of graph 706 between t22 and t23) up to the threshold engine speed705, and thereafter further spinning the engine to increase engine speedat a second, lower rate (as determined by the slope of graph 706 betweent24 and t25), for example, up to a higher threshold engine speed 707.Thereafter (after t25), vehicle speed may be maintained (at the elevatedvehicle speed) by maintaining the engine speed while also maintainingthe rotational speed of the transmission component.

In this way, when driving a hybrid vehicle in the electric-only modewith the engine spinning unfueled, by spinning the engine faster untilthe engine speed is at the threshold engine speed, and then spinning theengine slower after the engine speed is at the threshold engine speed,and while positioning the cam phaser after the engine speed is above thethreshold engine speed, the increase in engine speed may be coordinatedwith cylinder valve operation adjustments to reduce engine pumpinglosses. By reducing the pumping losses, the electric-only mode ofoperation may be prolonged enabling better vehicle fuel economy whilealso maintaining higher exhaust catalyst temperature.

As such, in response to a driver torque demand becoming higher than athreshold amount while the vehicle is in the electric-only mode, thetorque demand may be met by immediately shifting the vehicle to anengine-on mode of operation. During the engine-on mode of operationimmediately following the electric-only mode of operation, a controllermay reposition the cam phaser to increase airflow through the spinningengine. The position of the cam phaser may be adjusted based on arotational speed of the unfueled engine. In one example, the cam phasermay be returned from the second position enabling a lower airflowthrough the spinning engine to the first, default position enabling ahigher airflow through the spinning engine, and thereby better enablingan engine restart. Once the cam phaser has been repositioned to thesetting that improves vehicle operation in the engine-on mode, fuel andspark may be returned to the already spinning engine to further increasethe vehicle speed and the engine output.

It will be appreciated that while the example of FIG. 7 discusses anembodiment where the cam phaser is coupled to a mechanical pump, in analternate embodiment, the cam phaser may be coupled to an electric oilpump. Herein, adjusting a position of the cam phaser may includesubstantially immediately adjusting the position of the cam phaser withno delays. In such an embodiment, the engine speed may be increased assoon as the vehicle speed reaches the threshold vehicle speed, and theengine speed setting may be selected based on the vehicle speed andwithout taking into account the cam phaser position (for example, asshown in FIG. 5).

Now turning to FIG. 9, map 900 depicts example adjustments to a cylindervalve operation that may be performed during an engine-off mode withonly the electric motor driving the vehicle while a vehicle speed israised by increasing the speed of an unfueled engine. In the depictedexamples, intake and/or exhaust valve timing adjustments are shown. Eachvalve operation adjustment shows a valve timing and piston position,with respect to an engine position for a given engine cylinder.Specifically, the valve timings illustrated at 910-950 are compared toan engine position illustrated along the x-axis in crank angle degrees(CAD), and compared to a piston position along the y-axis with referenceto their location from top dead center (TDC) and/or bottom dead center(BDC), and further with reference to their location within the fourstrokes (intake, compression, power and exhaust) of an engine cycle atcurve 908. As indicated by sinusoidal curve 908, a piston graduallymoves downward from TDC, bottoming out at BDC by the end of the powerstroke. The piston then returns to the top, at TDC, by the end of theexhaust stroke. The piston then again moves back down, towards BDC,during the intake stroke, returning to its original top position at TDCby the end of the compression stroke.

At 910, a standard valve timing is depicted. In particular, curves 902and 904 depict valve timings for an exhaust valve (dashed curve 902) andan intake valve (solid curve 904) at a standard (unadjusted) valvetiming. As illustrated, an exhaust valve may be opened just as thepiston bottoms out at the end of the power stroke. The exhaust valve maythen close as the piston completes the exhaust stroke, remaining open atleast until a subsequent intake stroke has commenced. In the same way,an intake valve may be opened at or before the start of an intakestroke, and may remain open at least until a subsequent compressionstroke has commenced.

As a result of the timing differences between exhaust valve closing andintake valve opening, for a short duration, before the end of theexhaust stroke and after the commencement of the intake stroke, bothintake and exhaust valves may be open. This period, during which bothvalves may be open, is referred to as a positive intake to exhaust valveoverlap 906 (or simply, valve overlap), represented by a hatched regionat the intersection of curves 902 and 904. In one example, the positiveintake to exhaust valve overlap 906 may be a default position of theengine cams that is present during an engine restart (such as, an enginecold start). For example, the default position shown at 910 maycorrespond to the first position of a cam phaser enabling a higherairflow through the spinning engine (as discussed in the example of FIG.7). A controller may then reposition the cam phaser from the firstdefault position to a second, adjusted position while (or before) theengine is spinning unfueled to reduce engine pumping losses. At thesecond position of the cam phaser, the timing of the cylinder valves maybe adjusted (e.g., advanced or retarded) relative to the first positionto enable a lower airflow through the spinning engine.

As such, the cylinder valve operations may be adjusted to reduce flowrestrictions, and thereby reduce pumping losses. For example, byincreasing valve overlap, the airflow restriction and correspondingpumping loss may be reduced. As another example, by increasing thevalve-open dwell time while increasing the overlap (e.g., using VVTcapability on both the intake and the exhaust valves), pumping lossesmay be significantly dropped because air may move through both intakeand exhaust valves anytime air enters or exits the cylinders.

In one example, the cam phaser may be repositioned to the secondposition shown at 920 to only adjust an intake valve timing. Inparticular, when the cam phaser is in the second position shown at 920,an intake valve timing (solid curve 924) is advanced relative to thetiming when at the first position (shown at 910), while the exhaustvalve timing (dashed curve 922) is maintained. Due to the intake valvetiming adjustment, an amount of valve overlap 916 may also be increased.

In another example, the cam phaser may be repositioned to the secondposition shown at 930 to only adjust an exhaust valve timing. Inparticular, when the cam phaser is in the second position shown at 930,an exhaust valve timing (dashed curve 932) is retarded relative to thetiming when at the first position (shown at 910) while the intake valvetiming (solid curve 934) is maintained. Due to the exhaust valve timingadjustment, an amount of valve overlap 936 may also be increased.

In yet another example, the cam phaser may be repositioned to the secondposition shown at 940 to equally adjust each of an exhaust valve timingand an intake valve timing. In particular, when the cam phaser is in thesecond position shown at 940, each of the intake valve timing (solidcurve 944) and an exhaust valve timing (dashed curve 942) are equallyretarded at the second position relative to the first position. Due tothe dual equal retard adjustment, an amount of valve overlap 946 may bedecreased. While the depicted example shows dual equal retard, it willbe appreciated that in an alternate example, each of the intake valvetiming and exhaust valve timing may be equally advanced relative totheir timings in the first position.

In still another example, the cam phaser may be repositioned to thesecond position shown at 950 to adjust each of an exhaust valve timingand an intake valve timing independently. In particular, when the camphaser is in the second position shown at 950, the intake valve timing(solid curve 954) may be advanced while an exhaust valve timing (dashedcurve 942) is retarded at the second position relative to the firstposition. In the depicted example, the intake valve is advanced by thesame amount that the exhaust valve is retarded. However, in alternateexamples, the amount of intake valve advance may be different from theamount of exhaust valve retard. In still other examples, the intakevalve may be retarded while the exhaust valve is advanced, the amount ofintake valve retard being the same as, or different from, the amount ofexhaust valve advance. Due to the intake and exhaust valve timingadjustment, an amount of valve overlap 956 may be increased.

It will be appreciated that in the embodiment where the intake valve andthe exhaust valve can be adjusted independently, still other adjustmentsmay be possible. For example, each of the intake valve timing and theexhaust valve timing may be advanced, albeit by different amounts.Likewise, each of the intake valve timing and the exhaust valve timingmay be retarded, albeit by different amounts.

It will also be appreciated that while the examples of FIG. 9 depictadjustments in cylinder valve timing to illustrate cylinder valveoperation adjustments, this is not meant to be limiting. In alternateexamples, one or more of a cylinder (intake and/or exhaust) valvetiming, a cylinder valve lift, a duration of valve opening, a valve-opendwell time, and an amount of valve overlap may be adjusted. Likewise, toadjust the valve timing, a timing of opening the valve and/or a timingof closing the valve may be adjusted.

For example, at 950 in FIG. 9, the amount of overlap allows some of theexhaust air (non-combustion air) to exit through the intake manifoldduring the exhaust stroke, and some of the intake air to enter throughthe exhaust manifold during the intake stroke. Consequently, the totalair flow into and out of the cylinder may not change, but the net airexchange from intake to exhaust manifolds may be reduced, which may helpmaintain a higher exhaust catalyst temperature. In another example (notshown) the overlap shown in 950 may be further spread over the intakeand exhaust strokes by increasing the valve open dwell time. This wouldfurther reduce air transfer from intake to exhaust and also reduceairflow restrictions.

In this way, cylinder valve operation adjustments may be used to reduceairflow through the spinning engine and airflow restrictions when theengine is spun unfueled during an engine-off mode of vehicle operation.By reducing the airflow restrictions, parasitic losses through theengine, such as pumping losses can be reduced. In addition, oxygenloading of an exhaust catalyst can be reduced while also allowing thecatalyst temperature to be maintained (e.g., above a light-offtemperature). This provides additional fuel economy benefits during asubsequent engine-on mode of vehicle operation. In the event that atorque demand increases and an engine-on mode of vehicle operation isnecessitated to meet the demand, cylinder valve operation may bereadjusted (e.g., via cam phaser repositioning) to enable a higherairflow through the engine, thereby improving an engine restart.

It will also be appreciated that the disclosed approach may also allow ahybrid electric vehicle to operate in the electric mode with fewerengine-on disruptions until the system battery is depleted. As such,this may improve the driver's electric vehicle driving experience.

In this way, higher vehicle speeds may be achieved while a hybridvehicle is operated in an electric mode with only an electric motordriving the vehicle wheels. In addition, the time before an engineoperation is started in a hybrid electric vehicle can be increased andthe engine start delayed. By increasing the rotational speed of anengine when raising a vehicle speed, without supplying fuel to theengine, a rotating transmission component may be maintained belowrotational speeds at which the component may be degraded. At the sametime, cylinder valve operation adjustments may be advantageously used toreduce airflow, and resultant pumping losses, through the spinningengine. By enabling higher vehicle speeds to be attained withoutdegrading transmission components, the operator's drive experience inthe electric mode can be improved. By reducing pumping losses throughthe spinning unfueled engine, the electric mode of vehicle operation canbe prolonged to improve vehicle efficiency and fuel economy.

Note that the example control and estimation routines included hereincan be used with various system configurations. The specific routinesdescribed herein may represent one or more of any number of processingstrategies such as event-driven, interrupt-driven, multi-tasking,multi-threading, and the like. As such, various actions, operations, orfunctions illustrated may be performed in the sequence illustrated, inparallel, or in some cases omitted. Likewise, the order of processing isnot necessarily required to achieve the features and advantages of theexample embodiments described herein, but is provided for ease ofillustration and description. One or more of the illustrated actions,functions, or operations may be repeatedly performed depending on theparticular strategy being used. Further, the described operations,functions, and/or acts may graphically represent code to be programmedinto computer readable storage medium in the control system.

Further still, it should be understood that the systems and methodsdescribed herein are exemplary in nature, and that these specificembodiments or examples are not to be considered in a limiting sense,because numerous variations are contemplated. Accordingly, the presentdisclosure includes all novel and non-obvious combinations of thevarious systems and methods disclosed herein, as well as any and allequivalents thereof.

The invention claimed is:
 1. A method of operating a hybrid vehiclesystem, comprising: during an engine-off mode with only an electricmotor driving a wheel of the vehicle, raising vehicle speed whiledecreasing or maintaining a rotational speed of a rotating planetarygear transmission component by raising speed of the wheel by providing adrive force from the electric motor to the wheel while increasing speedof an unfueled engine coupled to the wheel while decreasing ormaintaining a rotational speed of a rotating planetary gear transmissioncomponent; and adjusting a cylinder valve operation based on the enginespeed.
 2. The method of claim 1, wherein the cylinder valve is an intakeand/or an exhaust valve, and wherein adjusting a cylinder valveoperation includes adjusting one or more of a valve timing, valve lift,duration of opening of the valve, and an amount of valve overlap.
 3. Themethod of claim 2, wherein the cylinder valve is a cam-actuated valve,and wherein adjusting a cylinder valve operation includes adjusting aposition of a cam phaser coupled to the cylinder valve.
 4. The method ofclaim 3, wherein adjusting the position of the cam phaser includesadvancing or retarding the cam phaser from a first position havinghigher air flow through the spinning engine to a second position havinglower air flow through the spinning unfueled engine.
 5. The method ofclaim 4, wherein the cam phaser is coupled to an electric oil pump andwherein the adjusting a position of the cam phaser includes immediatelyadjusting the position of the cam phaser.
 6. The method of claim 4,wherein the cam phaser is coupled to a mechanical oil pump and whereinthe adjusting a position of the cam phaser is delayed until an output ofthe mechanical oil pump is above a threshold pressure.
 7. The method ofclaim 6, wherein the delaying includes maintaining the cam phaser at thefirst position until the engine speed of the unfueled engine is above athreshold engine speed at which the output of the mechanical oil pump isabove the threshold pressure, and shifting the cam phaser to the secondposition after the engine speed is above the threshold engine speed. 8.The method of claim 7, wherein increasing the speed of the unfueledengine includes, spinning the engine to increase engine speed at afirst, higher rate up to the threshold engine speed, and thereafterfurther spinning the engine to increase engine speed at a second, lowerrate.
 9. The method of claim 8, wherein an intake valve timing isadvanced at the second position relative to the first position.
 10. Themethod of claim 8, wherein an exhaust valve timing is retarded at thesecond position relative to the first position.
 11. The method of claim8, wherein each of an intake valve timing and an exhaust valve timingare equally retarded at the second position relative to the firstposition.
 12. The method of claim 8, wherein an intake valve timing isadvanced and an exhaust valve timing is retarded at the second positionrelative to the first position.
 13. The method of claim 1, wherein thehybrid vehicle system further includes a battery electrically coupled toa generator, and wherein increasing the speed of the unfueled engineincludes spinning the engine unfueled using the battery via thegenerator, the engine speed increased in proportion to the raising ofthe vehicle speed above a threshold vehicle speed.
 14. A method ofoperating a vehicle system, comprising: during an electric-only mode ofoperation, raising vehicle speed above a threshold vehicle speed, whilespinning an engine coupled to a driven wheel unfueled using a battery,by providing a drive force to the wheel from a motor coupled to thewheel; and positioning a cam phaser coupled to intake and/or exhaustvalves of the engine to reduce airflow through the spinning engine. 15.The method of claim 14, wherein spinning the engine unfueled includesspinning the engine faster until the engine speed is at the thresholdengine speed, and spinning the engine slower after the engine speed isat the threshold engine speed, and wherein the positioning includespositioning the cam phaser after the engine speed is above the thresholdengine speed.
 16. The method of claim 15, further comprising, during anengine-on mode of operation immediately following the electric-only modeof operation, repositioning the cam phaser to increase airflow throughthe spinning engine; and returning fuel and spark to the alreadyspinning engine to further increase the vehicle speed and an engineoutput.
 17. A vehicle system, comprising: an engine; a cam phasercoupled to intake and/or exhaust cams of the engine; a transmissionincluding a planetary gearset and one or more gear elements; a batterycoupled to each of a motor and a generator, the engine, motor andgenerator each coupled through the transmission to a driven wheel; and acontroller with code for, during an engine-off mode with only the motordriving the vehicle via the wheel, raising vehicle speed above athreshold vehicle speed by providing a drive force from the motor to thewheel while spinning the engine unfueled; and adjusting a position ofthe cam phaser based on a rotational speed of the unfueled engine. 18.The vehicle system of claim 17, wherein the adjustment includesadvancing or retarding the position of the cam phaser from a firstposition enabling a higher airflow through the spinning engine to asecond position enabling a lower airflow through the spinning engine,the second position based on the rotational speed of the engine.
 19. Thesystem of claim 18, wherein a rotating component of the transmission ismaintained at or below a threshold rotational speed while raising thevehicle speed above the threshold vehicle speed.